1. Field of the Invention
This invention relates to capacitor pickoff circuits.
2. Prior Art
A capacitive pick-off position detector is a position sensor which encodes displacement as a function of the capacitance value of a variable capacitor. Differential capacitance pickoffs encode displacement as a function of the difference between the capacitance values of two variable capacitors, which are typically ganged together so that as one capacitor increases in value, the second decreases in value. Typically, each of the differential capacitors is positioned on opposite sides of a moving mass or pendulum. It is desired that an electrical signal be provided which is a linear function of the pendulum displacement.
Prior art capacitive pick-off circuits use amplitude-modulation to encode variations in capacitance for a pair of differential capacitors. Each of the capacitors of a pair is used to amplitude modulate a respective reference carrier. The instantaneous amplitude of each of the modulated signals is proportional to the instantaneous value of each capacitor. The amplitude of each of the modulated signals is detected using, for example, a diode detector. The detected signals are then subtracted to provide an output signal which is proportional to the displacement of the pendulum. Signal processing with this prior art approach involves taking differences between two relatively large signals to produce a difference signal, which is very small. Subsequently, the small difference signal is greatly amplified. This approach introduces DC errors before appreciable signal gain is obtained. For a closed-loop, servoed-type of measuring system, DC errors can introduce unacceptable mechanical offset errors. For example, large DC errors can cause the servo to shift the equilibrium pendulum position away from its ideal mid-point position.
A prior art demodulator for a DSB-SC signal includes an adder circuit which adds the original DSB-SC signal to be demodulated with a second signal. The second signal is formed by first inverting and doubling the original signal and then chopping the resultant signal with a square wave at the frequency of the DSB-SC carrier reference signal. The resultant chopped signal is then added to the original signal to provide an output baseband signal which includes the information modulated into the original DSB-SC signal. For good performance, precise matching of gain setting components is required, as in most prior art demodulators.
A circuit which measures variations in the capacitance of a single measuring capacitor is disclosed in the Rudolf U.S. Pat. No. 4,483,194, which describes an acceleration measuring circuit. The measuring circuit is associated with a variable capacitance formed by a flap, or vane, and an electrode. The variable capacitance is part of a measuring-bridge circuit which also includes a fixed capacitance and two resistors. A high-frequency signal is injected into the measuring-bridge circuit at the junction of the two capacitances. The junction point of the two resistors is connected to ground. The junction of the variable capacitance and a first one of the resistors is coupled to an input terminal of a first rectifier circuit. The junction of the fixed capacitance and the second resistor is coupled to an input terminal of a second rectifier circuit. The output terminal of the first rectifier circuit is connected to the non-inverting input terminal of a differential amplifier, while the output terminal of the second rectifier circuit is connected to the inverting input terminal of the differential amplifier. Movement of the flap under the effect of acceleration produces corresponding amplitude modulation of the signal at the junction of the variable capacitance and the first one of the resistors. The differential amplifier measures the difference in amplitude between the two rectified signals and provides an output signal which represents the acceleration of the flap. This patent also describes using the acceleration circuit in a feedback arrangement to hold the flap in a position close to its equilibrium position.
The Holdren et al. U.S. Pat. No. 4,165,483 describes a pair of differential capacitors. A reference current generator applies a square wave to each pickoff capacitor and the resulting voltage across each pickoff capacitor is then applied to a fixed capacitor. The resulting fixed capacitor currents each respectively provide a measure of the capacitance of the respective pick-off capacitors.
The Holdren et al. U.S. Pat. No. 4,206,400 describes a capacitive pickoff used in a servoed accelerometer, where a loop feedback circuit generates a currents through a torque coil sufficient to move a measuring pendulum to a midpoint between the capacitor plates.